Carboxylate alumoxanes, also known as carboxylato alumoxanes, are inorganic-organic hybrid materials that contain a boehmite-like ([AlO(OH)]n) aluminum oxygen core (FIG. 1), to whose surfaces are attached covalently bound carboxylate groups (i.e., RCO2−, where R=alkyl or aryl group) (FIG. 2). The carboxylate groups are tethered to the aluminum-oxygen surface through bidentate bonding of the carboxylate group to two aluminum atoms on the surface of the boehmite particle. The properties and processability of the carboxylate alumoxanes are strongly dependent on the nature and size of the attached organic groups.
Until recently, carboxylate alumoxanes were not ideal as processable precursors because they were relatively difficult to prepare. New synthetic routes, including aqueous based techniques, have been discovered which have reduced the high cost of preparing the carboxylate alumoxanes.
Although there can be significant advantages to the synthesis of carboxylate alumoxanes via the sol-gel route (such as preparation of small particles size and good homogeneity), however, a number of significant difficulties remain. For example, while the reaction of boehmite with a carboxylic acid can be carried out in either water or a variety of organic solvents (including, but not limited to toluene and xylene), it is preferable to use water as the solvent so as to minimize the production of environmentally problematic waste. In a typical reaction, the carboxylic acids are added to boehmite or pseudoboehmite particles, the mixture is heated to reflux at temperatures as high as 100° C. for a period of time, often greater than 24 hours. The water is removed and the resulting solids are collected. As can be appreciated, this process may generate excessive waste, is time consuming, and requires elevated reaction temperatures, and typically must be performed on a batch process.
It is desirable, therefore to identify materials and processes that can be continuously used to prepare carboxylate alumoxanes using lower reaction temperatures and shorter reaction times. The process would preferably lend itself to the fabrication of a wide range of carboxylate alumoxanes with fine control over composition and particle size. The present invention provides a method of making carboxylate alumoxanes that provides these benefits and avoids many of the problems of prior art processes.